Characterization of hydrate formation in drilling muds using Differential Scanning Calorimetry (DSC)
Résumé
During the drilling operation, a viscous fluid called drilling mud is injected inside the tubing and lifts the solids cuttings to the surface through the annular space between the tubing and the formation. Drilling muds may be water based (brine solutions of clays and polymers) or oil based (invert emulsion of brine in oil). With the increasing number of deep offshore drilling operations, operators and service companies are confronted to more and more complex technical challenges. Extreme conditions encountered at these depths require an adaptation of the drilling muds. In particular, range of temperature and pressure (up to –1 °C and 400 Bars) are favorable conditions to the formation of gas hydrates. Hydrates are solid structures formed from water and gas: water contained in drilling muds will form, under certain temperature and pressure conditions, a solid cage which entraps the gas molecules coming from the reservoir. Formation of these solid gas hydrates is liable to plug the lines as well as the annular, and may cause interruption of the drilling operation and even destruction of rig equipment. Deep offshore drilling operators are aware of this problem and some operational solutions and drilling mud formulations are proposed and utilized, but when extreme water depths are attained, classical inhibitors solutions alone may be ineffective. The usual way to determine the thermodynamic conditions of the formation of hydrates in drilling mud formulations is to use PVT cell. This technique requires heavy instrumentation and often does not permit to work with a whole formulation (specially in the presence of solids). Moreover, PVT cells do not give a quantitative evaluation of the kinetic properties of hydrate formation.
An innovative methodology was elaborated using Differential Scanning Calorimetry (DSC) to determine the thermodynamic equilibrium properties and kinetics of hydrate formation in mud formulations, particularly in the presence of large amounts of mineral. This technique allows the measurement of heat transfers as a function of time, temperature and pressure and thus detects phase transitions. Using DSC, hydrate appearance in several drilling muds formulations (WBM or OBM) is shown and quantified. Validation with PVT cell measurement and investigation of the influence of several additives (salts, solids) are performed. The influence of hydrocarbon composition i.e. methane or natural gas on hydrate formation is studied for different mud continuous phases. A methodology using isothermal experiments to quantify kinetics of hydrate formation is proposed. The effectiveness of several inhibitors is also measured. Through this work, DSC has proven to be an efficient and easy to use technique for characterizing hydrate formation in drilling muds.